The present invention is directed to overvoltage protectors of the type for use in telecommunication lines. In one aspect, the invention relates to a gas tube based protector that is operable without the need for a backup varistor or other device.
Overvoltage protectors are known that use a gas tube as the primary means for diverting voltage surges to ground and also use a second device, such as a metal oxide varistor (MOV), as a backup protection device. The backup protector is intended to become operable in the event of or to prevent a failure in the gas tube, as might occur from a gas leak from the tube. Arrangements of this type are described in U.S. Pat. Nos. 5,909,349 and 5,388,023. The MOV is intended to become operable at higher voltages than the gas tube so that the gas tube will operate to ground a surge rather than the MOV under normal circumstances.
Unfortunately, there are drawbacks to using an MOV as a backup device. First, it may be possible for a defective MOV to be a short rather than act as a varistor. Also, the use of MOVs drives up the cost of making the overvoltage protector.
U.S. Pat. No. 5,388,023 teaches placement of a cylindrical non-conductive disk, rather than an MOV, between the end cap and the end of the gas tube in order to ensure that there is no contact between the end caps and the outside body of the gas tube. When the protector assembly becomes overheated, as in the case of a malfunction, solder disks near the axial ends of the gas tube melt, and it is intended that a spring clip will urge the end caps against the end terminals of the gas tube, thereby intentionally shorting out the protector assembly and rendering it inoperable. This is intended to provide a xe2x80x9cfail-safexe2x80x9d mode for the protector assembly. In practice, however, the mass of the molten solder may not flow adequately and continue to prevent the end caps from contacting the end terminals of the gas tube, thereby preventing the assembly from providing the fail-safe protection.
Other problems with conventional protector assemblies stem from the fact that spring clips used to secure the end caps against the ends of the gas tube can apply forces to the caps unevenly, thereby causing the caps to tilt or become seated improperly. The outer axial surface of the cap is usually flat, and the portions of the spring clips that engage them are also typically flat. However, the solder pellet and MOV located inside of the cap have a smaller diameter than that of the cap, allowing the cap room to move about. Uneven application of axial forces by the spring clip presents a small possibility that the cap could contact conductive portions of the gas tube and essentially short out the protector assembly prematurely.
The present invention provides an overvoltage protector assembly that utilizes a gas tube to divert surges to ground. An exemplary protector assembly is described that includes a gas tube that is the primary and only mechanism for transmitting excessive voltage to ground.
The protector assembly has several features that ensure reliability of the gas tube as a protective element. A toroidal non-conductive element, such as a glass, ceramic, or plastic insert, is disposed between the end cap and the axial ends of the gas tube. A fusible element formed of a fluxed solder pellet is also located between the end cap and the axial ends of the gas tube. The end cap provides includes a number of apertures therein and a raised projection on its outer surface. The spring clip of the protector element engages the projection of the cap so that axial forces applied by the clip are centrally applied to the cap and not unevenly distributed on different portions of the cap.
In the event of a failure in the gas tube that would result in melting of the fluxed solder pellet, the molten solder will adhere to the surfaces of the end cap and the non-conductive element due to the presence of the flux in the solder. Solder from the pellet is transmitted through the apertures of the end caps as well as the hole in the toroidal non-conductive element. As a result, the mass of the solder that provided for a gap between the end caps and the end conductors of the gas tube essentially disappears, and the spring clip urges the end caps against the conductors. Molten solder that flows into the hole in the toroidal element will also be transmitted to the end conductors of the gas tube thereby providing an electrical contact between the end cap and the conductors of the gas tube.
The protector assembly also uses end caps having raised projections that engage the tangs of a securing clip. In contrast to the prior art assemblies, this arrangement ensures that the force applied to the caps by the clip is centralized and does not result in the end caps being tilted onto the ends of the gas tube, thereby resulting in a premature short-out condition.
The improvements of the present invention help provide an effective and reliable protector assembly that does not require an MOV back up device. The devices of the present invention also reduces the costs associated with creating protector assemblies and improve overall reliability.